1. Field of the invention
The present invention relates to a testing apparatus for detecting an image signal in radio waves leaking from an information processing system such as a computer system. More particularly, the present invention is concerned with a testing apparatus for detecting an image signal in radio waves leaking from an information processing system that includes an image signal detecting device which detects an image signal to be displayed on a visual display device included in the information processing system by checking radio waves leaking out from the information processing system, and an image reproducing device which reproduces the image to be displayed on the visual display device using the detected image signal, and that performs a test to see if an image to be displayed on the visual display device can be intercepted and viewed by stealth.
2. Description of the Related Art
In general, a plurality of radio waves, which are modulated according to the timing for transmitting an image signal, can leak out from a display unit of a computer system, other units coupled to the computer system, or connecting cables thereof. Though very weak, these leakage radio waves contain image information. Accordingly, if such radio waves are intercepted by using a high-sensitivity antenna or receiver by a third party, the third party can then process the waves to reproduce image information. This leads to information leakage. A testing apparatus for detecting an image signal in a radio wave leaking from an information processing system is used for checking if an image signal can be intercepted and if the image can be reproduced by stealth.
Accordingly, it is desired to provide an effective apparatus for checking whether or not image information contained in leakage radio waves existent in an information processing system such as a computer system may be intercepted for viewing by stealth.
In the past, a testing apparatus for detecting an image signal in radio waves leaking from an information processing system, shown in FIG. 1, has been available.
As shown in FIG. 1, reference numeral 1 denotes an information processing system to be tested for leakage radio waves containing an image signal and 3 denotes a testing apparatus for detecting an image signal in radio waves leaking from the information processing system 1.
The information processing system 1 includes a personal computer 11 and a visual display unit such as a CRT (cathode ray tube) connected by a connecting cable 11a. Radio waves containing an image signal are leaking from the VDU (visual display unit) 2 or the connecting cable 11a.
The conventional testing apparatus 3 for detecting an image signal in radio waves leaking from an information processing system 1 comprises an antenna 4, a pre-amplifier 13, a receiver and visual display unit 5, a vertical synchronizing signal oscillator 82, and a horizontal synchronizing signal oscillator 81. The antenna 4 detects the radio waves leaking from the information processing system 1. The receiver and visual display unit 5, which can be made by reorganizing a television set, reproduces the image to be displayed on the VDU 2 of the information processing system 1 from the detected image signal. The horizontal synchronizing signal oscillator 81 generates a horizontal synchronizing signal which is required to synchronize the horizontal image scanning in the information processing system 1. The vertical synchronizing signal oscillator 82 generates a vertical synchronizing signal which is also required to synchronize the image scanning in the information processing system 1. The phase of the horizontal synchronizing signal oscillator 81 and the vertical synchronizing signal oscillator 82 is adjusted by an operator to reproduce the detected image.
The conventional testing apparatus 3 for detecting an image signal in radio waves leaking from an information processing system 1 is operated as described below.
The personal computer 11 is leaking out radio waves that are modulated in amplitude according to screen information. The radio waves do not contain any synchronizing signal. An ordinary television broadcast wave contains both an image signal and a synchronizing signal.
Radio waves received by the antenna 4 are amplified by a pre-amplifier 13, and then fed to an antenna terminal of the receiver and visual display unit 5. The receiver and visual display unit 5 processes the radio wave according to a procedure similar to that for an ordinary television broadcast wave. Since no synchronizing signal is included, a synchronizing signal must be supplied.
The horizontal and vertical synchronizing signal oscillators 81 and 82 supply synchronizing signals adjusted by an operator. The adjusted synchronizing signals are further adjusted to be fully consistent in frequency and phase with those in the personal computer 11, whereby a display screen of the personal computer 11 is reproduced.
In the conventional testing apparatus 3 for detecting an image signal in radio waves leaking from an information processing system 1, if only leakage radio waves can be received, image information of the computer system 1, which is composed of the personal computer 11 and VDU 2, can be intercepted for viewing by stealth. To reproduce an image, the synchronizing signals generated by the horizontal synchronizing signal oscillator 81 and vertical synchronizing signal oscillator 82 must be adjusted so that the synchronizing signals will be precisely consistent in frequency and phase with those used by the computer system 1.
This adjustment is rather hard or complicated to do for an ordinary person. Accordingly, this testing must be entrusted to a specialized agency having advanced technologies for stealth-view interception.
Even when precise synchronizing signals are generated, it is impossible to fully accord the phase of the generated signals with those in the information processing system 1. The display positions of reproduced screens shift as time passes.